Quantcast Bioengineering by Zones (Cont.)

 
  
 
Appendix B: Bioengineering for Streambank Erosion Control -- Guidelines
Bank zone. That portion of the bank usually above the normal high-water level; yet, this
site is exposed periodically to wave-wash, erosive river currents, ice and debris movement,
and traffic by animals or man. The site is inundated for at least a 60-day duration once every
two to three years. The water table in this zone frequently is close to the soil surface due to
its closeness to the normal river level.
In the bank zone, both herbaceous (i.e., grasses, clovers, some sedges and other herbs)
and woody plants are used. These should still be flood tolerant and able to withstand partial
to complete submergence for up to several weeks. Allen and Klimas (1986) list several grass
and woody species that can tolerate from 4 to 8 weeks of complete inundation. This list,
should not be considered exhaustive, however. Whitlow and Harris (1979) provide a listing
of very flood-tolerant woody species and a few herbaceous species by geographic area within
the United States that can be used in the bank zone.
Skeesick and Sheehan (1992) report on several other herbaceous and woody plants that
can withstand tens of feet of inundation over 3 to 4 months in two different reservoir
situations in Oregon. These same species are often found along streambanks. Local
university botanists and plant material specialists within the NRCS should be consulted when
seeking flood-tolerant plants . Various willows can be used in this zone, but they should be
shrublike willows such as sandbar willow (S. exigua) and basket willow (S. purpurea var.
nana). Edminster et al. (1949) and Edminster (1949) describe successful use of basket willow
for streams and rivers in the Northeast. Shrub-like willow, alder, and dogwood species have
been used in Europe successfully (Seibert 1968). Red-osier dogwood (Cornus stolonifera)
and silky dogwood ( C. amomum) also have been used in the Northeast (Edminster et al. 1949
and Edminster 1949). Seibert (1968) notes that in periods of high water, the upper branches
of such shrubs reduce the speed of the current and thereby the erosive force of the water.
The branches of these have great resilience, springing back after currents subside.
Terrace zone. That portion of the bank inland from the bank zone; it is usually not
subjected to erosive action of the river except during occasional flooding. This zone may
include only the level area near the crest of the unaltered "high bank" or may include sharply
sloping banks on high hills bordering the stream.
The terrace zone is less significant for bank protection because it is less often flooded, but
can be easily eroded when it is flooded if vegetation is not present. Vegetation in this zone
is extremely important for intercepting floodwaters from overbank flooding, serving to reduce
super-saturation and decrease weight of unstable banks through evapotranspiration processes
and for tying the upper portion of the streambank together with its soil-binding root network.
Coppin and Richards (1990) provide a detailed explanation of plant evapotranspiration, but
summarize by saying, " Apart from increasing the strength of soil by reducing its moisture
content, evapotranspiration by plants reduces the weight of the soil mass. This weight
reduction can be important on vegetated slopes where the soil may be potentially unstable."
As denoted in Figure 6, the terrace zone can contain native grasses, herbs, shrubs, and
trees that are less flood tolerant than those in the bank zone, but still somewhat flood tolerant.
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